Shock insensitive transducer

ABSTRACT

A microphone with a transducer having a vibratal diaphragm at one end defining one of the walls of a diaphragm chamber and a shock mount diaphragm at the other end defining one of the walls of a cavity, the diaphragm chamber and cavity being acoustically interconnected, and the transducer being mounted on the microphone casing for translation along the axis normal to the diaphragm, the shock mount diaphragm being connected to the microphone casing, whereby a shock or vibration impressed upon the microphone casing with a component normal to the diaphragm changes the size of the cavity to produce a pressure which is transmitted to the diaphragm cavity to oppose inertial deflection of the diaphragm responsive to the shock or vibration.

United States atent [191 Watson [451 Oct. 16, 1973 SHOCK INSENSITIVE TRANSDUCER [75] Inventor: Alan Reed Watson, Niles, Mich.

[73] Assignee: Electro-Voice Incorporated, Berrien,

Mich.

[22] Filed: June 15, 1972 [21] Appl. No.: 263,260

[52] U.S. Cl l7 9/ll5.5 R, 179/146 R, 179/184, 179/179 [51] Int. Cl H04m l/04, H04r 9/00 [58] Field of Search 179/184, 116, 123, 179/132, 139, 146 R, 147, 148 R, 150, 179

[56] References Cited UNITED STATES PATENTS 3,436,495 4/1969 Gorike 179/146R I FOREIGN PATENTS OR APPLICATIONS 1,121,718 7/1968 Great Britain 179/146 Primary Examiner-Kathleen I-I. Claffy Assistant Examiner-Thomas L. Kundert Attorney-Marshall A. Burmeister et al.

[ 5 7] ABSTRACT casing with a component normal to the diaphragm.

changes the size of the cavity to produce a pressure which is transmitted to the diaphragm cavity to 0ppose inertial deflection of the diaphragm responsive to the shock or vibration.

10 Claims, 3 Drawing Figures PATENIEDUBI 161915 3.766.333

SHEET 10F 2 FIG-.l

1 SHOCK INSENSITIVE TRANSDUCER This invention relates to a transducer which is relatively insensitive to shock. More particularly, it relates to a microphone which produces an electrical signal responsive to acoustical energy and which produces little electrical output responsive to shock impressed upon the microphone.

Almost all microphones currently in use utilize a diaphragm which is coupled to an electromechanical transducer. The electromechanical transducer itself may employ a piezoelectric element for generating electrical potentials responsive to deflection, or may be a dynamic or moving coil type of microphone. The moving coil microphone employs a small coil mounted on the diaphragm, called a voice coil, and the voice coil is suspended in a narrow cylindrical gap in a magnetic circuit. In either event, the diaphragm moves relative to a stationary structure.

Any shock impressed upon the stationary structure will result in movement of the diaphragm relative to the stationary structure due to the inertia of the diaphragm and the other elements in its moving system. Hence, a shock impressed upon the stationary structure of such a microphone will result in an electrical output. This is also true of other types of vibration measuring transducers.

It is an object of the present invention to provide an electromechanical transducer which minimizes electrical response to shock impressed upon the stationary structure of the transducer. More specifically, it is an object of the present invention to provide a microphone in which mechanical means are employed to produce an acoustical signal which substantially cancels the force impressed by shock applied to the stationary structure on the diaphragm of the microphone.

Prior to the present invention, efforts have been made to isolate microphones from sources of shock waves by means of cushioning mountings. U.S. Pat. No. 2,920,150 entitled SUPPORT FOR MICRO- PHONES" by Louis R. Burroughs and U.S. Pat. No. 3,155,780 entitled MICROPHONE AND MOUNT- ING MEANS ASSEMBLY by Louis R. Burroughs are examples of cushioning mounting structures for isolating microphones. Another approach to avoiding the inertia induced electrical output of a microphone responsive to shock is set forth in U.S. Pat. No. 2,835,735 of Carl J. Moen entitled ANTI-SHOCK TRANS- DUCER in which a second electromechanical transducer is mounted within the microphone housing and isolated from the acoustical field to produce an electrical signal responsive to shock impressed upon the stationary structure of the microphone. The electrical signal thus produced responsive to shock is used to cancel the shock induced output of the acoustically responsive transducer of the microphone.

The use of cushioning mountings to eliminate shock induced output from microphones is of limited effectiveness. The construction disclosed is the Moen patent requires a second transducer, is substantially more costly to construct, and results in an additional impedance in the output circuit of the microphone with a resulting loss of sensitivity.

It is an object of the present invention to provide an electromechanical transducer which is less sensitive to shock than those devices relying upon cushioning mountings for shock isolation and which overcomes the disadvantages inherent in a two transducer cancellation system as described above.

More specifically, it is an object of the present invention to provide a microphone which incorporates mechano-acoustical means for opposing deflection of the microphone diaphragm with respect to the stationary structure of the microphone which tends to result from inertial reactions to shock impressed upon the microphone. I

In accordance with the present invention, the transducer of the microphone is compliantly mounted with respect to the microphone casing so that a shock impressed upon the micrphone casing will result in a displacement of the transducer with respect to the microphone casing. The displacement of the transducer with respect to the microphone casing is utilized to vary the size of a chamber within the transducer which is coupled to the diaphragm of the microphone, and arranged to generate a pressure on the rear side of the microphone diaphragm which opposes motion of the diaphragm resulting from the inertia of the diaphragm under impressed shock or vibration.

The present invention and further objects thereof will become apparent upon a further consideration of this specification, particularly when viewed in the light of the drawings, in which:

FIG. 1 is a diagrammatic view of a microphone constructed according to the teachings of the present invention;

FIG. 2 is an exploded isometric view of the transducer for a microphone constructed according to the teachings of the present invention; and

FIG. 3 is a longitudinal sectional view of a microphone employing the transducer of FIG. 2.

The invention may be most readily understood in connection with the diagrammatic view of FIG. 1. FIG. 1 illustrates a microphone transducer 10 mounted within a casing 12, but it should be understood that the casing could conceivably be omitted if the transducer 10 were properly mounted on the structure with which it is to be utilized, such as a microphone stand or boom. However, since microphones are intended to'be used as portable devices in most cases, it is convenient to utilize a casing which provides thev proper mounting for the transducer. The transducer 10 is mounted on the casing 12 by a compliant ring 14 which surrounds the cylindrical transducer housing 16 and is secured on the interior surface of the tubular casing 12. In addition, one end'of the transducer 10 is secured on a closed end of the tubular casing 12 by a protruding anchor 18 as will be referred to hereinafter.

The transducer 10 is illustrated as a dynamic permanent magnet device, that is, a device containing a permanent magnet 20 with a yoke structure 22 which forms an air gap 24 which accommodates a voice coil 26 mounted on a diaphragm 28. The gap 24 communicates through the magnetic structure and a small aperature 30 with a large chamber 32 disposed within the housing 16 of the transducer 10, damping material 34 being disposed in the magneticstructure 32 between the aperture 30 and the voice coil gap 24.

The transducer has a shock mount diaphragm 36 mounted on the end of the housing 16 opposite the diaphragm 28, and the shock mount diaphram 36 is mounted on the anchor 18 of the casing 12 and acoustically sealed to the housing 16. A disc 38 is mounted within the tubular housing 16 adjacent to the shock mount diaphragm 36, and the disc 38 provides a mass to the system, as will be indicated hereinafter. In addition, the disc 38 has a passageway 40 communicating between the chamber 32 within the housing 16 and a variable volume cavity 42 disposed between the disc 38 and the shock mounting diaphragm 36.

The end of the casing 12 opposite the anchor 18 is open, and a protective screen 44 extends over this end. Hence, sound waves can pass through the screen 44 to impinge upon the diaphragm 28, thus producing an electrical current in the voice coil 26 in the conventional manner. However, any shock or vibration impressed upon the transducer with a component along the axis of the housing 16 will accelerate the diaphragm 28 with respect to the magnetic structure 20 due to the inertia of the moving system which consists essentially of the diaphragm 28 and voice coil 26. Acceleration of the moving system with respect to the magnetic structure 20, as in all dynamic microphones, results in an electrical output responsive to the shock or vibration impressed upon the transducer, and this principle is utilized in accelerometers to measure the shock or vibration.

In accordance with the present invention, however, a shock or vibration impressed upon the casing 12 of the microphone with a component parallel to the axis of the housing 16 results in deflection of the housing 16 with respect to the casing 12. The compliant mounting ring 14 and the shock absorber diaphragm 36 permit movement of the housing 16 with respect to the casing 12, and this movement results in a change in the volume of the chamber 42. The change in the volume of the chamber 42 responsive to shock or vibration changes the pressure within the chamber 42 in a manner to oppose the change in pressure in the region immediately behind the diaphragm, referred to as the diaphragm chamber and designated 46. The change in pressure in the variable volume chamber 42 is transmitted through the orifice 40, the chamber 32, the aperture 30, the sound absorbent material 34, 32, voice coil gap 24 to the diaphragm chamber 46, thus tending to cancel the shock induced inertial pressure change in the diaphragm chamber 46. Further, by proper design, the cancellation may be made independent of the magnitude of acceleration due to shock impulse.

The required conditions for producing a microphone in which cancellation of the inertial induced pressure change in the diaphragm chamber is independent of the magnitude of the shock or vibration impressed on the casing and can be determined from the following mathematical expressions derived for a simple impulse shock impressed upon the microphone casing. The force F which is applied to the anchor 18 of the shock mount diaphragm 36 as a result of shock induced deflection of the microphone case 12 is given by the following expression.

Where M, is the mass of the transducer, and a, is the acceleration of the transducer. The force F which is needed to resist motion of the diaphragm 28 relative to the magnetic structure under the shock condition is given by the following expression.

Where M, is the mass of the moving system, namely the voice coil and diaphragm, and a, is the acceleration of the moving system. If no relative motion is to occur between the magnet structure 20 and the voice coil 26, then a, must equal (1, Hence, the force required to resist motion of the diaphragm relative to the magnetic structure may be expressed as follows.

Since the force is conveyed from the shock mount chamber 42 to the diaphragm chamber 46 by air pressure acting over known surface areas, the forces can be expressed also in terms of pressure and area, as follows.

Where P is a pressure per unit area in the shock mount cavity 42, and A, is the surface area of the shock mount diaphragm 36. Further,

F2 Pz'A:

Where P is a pressure per unit area in the diaphragm chamber 46, and A is the surface area of the diaphragm. From these equations it is clear that Since the pressure in the diaphragm chamber 46 which is derived from the change in volume of the shock mount cavity 42 is the result of flow losses through the orifice 40, the chamber 32, the aperture 30 and the sound absorbent material 34, as well leaks to the outside atmosphere, the ratio of the pressure in the shock mount cavity 42 to the pressure in the diaphragm chamber 46 defines the needed loss relationship for cancellation. This expression reduces to the following form.

Hence, by proper selection of the mass of the transducer, the mass of the voice coil, the area of the surface of the shock mount diaphragm 36, and the area of the surface of the diaphragm 28, the microphone will cancel acoustically inertial response to shock, and this cancellation will be independent of the magnitude of the shock itself.

FIG. 2 and 3 illustrate a preferred construction of a microphone meeting the foregoing conditions. FIG. 3 illustrates the microphone assembly including a transducer 10A mounted within a casing 12A by means of a compliant mounting means including a ring l4A. In the particular construction, an intermediate shell 48 is affixed to the casing 12A and the ring 14A in order to utilize the transducer 10A in a plurality of different shaped and sized casings, although it is to be understood that the shell 48 is not necessary to the practice of the present invention. The shell 48, however, does perform an additional advantageous acoustical function in that it prevents sound waves from impinging upon the surface of the transducer 10A which could at low frequencies produce an undesirable directional characteristic.

The casing 12A has a central tubular portion 50 which is closed at one end by a semi-spherical end portion 52. The other end of the tubular portion 50 is provided with a recess 54 which is threaded on its outer surface, and engages a threaded ring 56 of a dome shaped screen 58, thus providing an opening for sound to enter the casing 12A.

The transducer is illustrated in exploded form in FIG. 2 and in section in FIG. 3. The transducer has a pot structure 60 which is cylindrical in shape, open at its one end and provided with a cylindrical orifice 62 in the top wall 64 at the other end. The cylindrical orifice 62 forms one of the surfaces of a magnetic gap as will be described hereinafter. The one end of the pot structure 60 rests upon a circular gasket 66 which is wedged between the pot structure 60 and a lower housing 16A of the transducer. The pot structure 60 is secured in position by an upper housing 168 which is cylindrical in form and extends about the exterior surface of the lower housing 16A. The upper housing 168 has an end wall 68 opposite the lower housing 16A which engages a recess 70 in the exterior surface of the pot structure 60 to secure the pot structure in position. The lower housing 16A has protruding flanges 72 which extend between protruding flanges 74 on the upper housing 163, the flanges having recesses 76 which accommodate a common ring 78 to secure the upper housing 163 on the lower housing 16A.

An end plate 80 is mounted within a recess 82 in the end of the pot structure 60 confronting the lower housing 16A, and the end plate 80 is provided with an aperture 84 adjacent to the periphery thereof. The surface of the end plate 80 opposite the lower housing 16A is provided with a flat indentation 86, and a cylindrical magnet 88 has one end thereof disposed in abutment wtih the recess 86 of the end plate 80, the other end of the magnet being disposed within the opening 62 in the wall 64 of the pot structure to form a magnetic gap designated 24A. The magnet 88 is maintained coaxially within the opening 62 by a centering ring 90 which is disposed within a cylindrical recess 92 on the interior surface of the pot structure 60, centering ring 90 having a plurality of spaced recesses 94 confronting its center to permit sound to pass between the magnet and the centering ring. Felt washers 96 are disposed between the centering ring 94 and the end plate 80 to provide an acoustical resistance in the path between the aperture 84 andthe magnetic gap 24A.

A diaphragm 28A is cemented at its periphery on the exposed surface of the wall 68 of the upper casing 168, and the diaphragm 28A carries a voice coil 26A which is disposed within the cylindrical magnetic gap 24A. A cover 98 has a depending cylindrial wall 100 which engages the periphery of the upper housing 16B. The cover also has a flat wall 102 provided with apertures 104 which confront the diaphragm 28A, and a protective screen 106 is cemented to the wall 102 opposite the diaphragm 28A.

The housing 16A is hollow and forms an interior chamber 32A which communicates with the aperture 84. At the sideof chamber 32A remote from the aperture, a mass is added to the transducer in the form of two discs 108 which are secured to the end of the lower housing 16A by means of a hollow eyelet 110 which extends through axial bores l 12 in the discs 108. The eyelet 110 extends into a conical recess 114 in the end of the lower housing 16A remote from the housing 16B, and a shock absorbent diaphragm 116 is mounted, on the lower housing 16A to form a shock diaphragm cavity 42A between the conical surface 114 and the shock mount diaphragm 116. The shock mount diaphragm 116 is a cup-shaped resilient plastic member with a cylindrical sidewall 118 which is secured, as by cementing, to the cylindrical outer end wall of the lower housing 16A. The shock mount diaphragm 116 also has a compliant disc-shaped wall 120 which closes the shock mount cavity 42A and which carries a protruding stem 122 which extends through an aperture 124 in the shell 48, thereby securing the transducer 10A on the shell 48. In addition, the stem 122 has an inwardly extending axial recess 124 which is provided with an eyelet 126, and'a pin 128 is mounted on the semi-spherical end 52 of the casing 12A and extends into the eyelet and is secured therein. In this manner, both the shell 48 and the transducer 10A are secured on the casing 12A.

In one particular construction of the microphone of FIGS. 2 and 3, the effective area of the diaphragm 28A is 0.50 square inch. The effective area of the rear shock mount diaphragm 116 (essentially the wall 120) is 0.188 square inches. The mass of the diaphragm-voice coil assembly is 120 milligrams, and the mass of the transducer assembly is 47 grams. The volume of the shock mount cavity 42A is 6 cubic centimeters, and the volume of the cavity between the discs 108 and the end plate is 60 cubic centimeters. The inner diameter of the port formed by the eyelet is 0.062 inch, and the length of the discs port formed by the eyelet 110 is 0.350 inch. The diameter of the diaphragm 28A is the 0804 "marine volume "of "the diaiihragm chamber (designated 46A) is approximately 0.08 cubic centimeters, and the diameter of the aperture 84 is fivesixteenths inch.

As previously stated, cancellation to shock can be achieved by adjusting the losses in acoustical energy transferred from the shock mount chamber 42A to the diaphragm chamber 46A. These losses are controlled by the diameter of the eyelet 110, the size of the chamber 32A between the discs 108 and the end plate 80', the size of the aperture 84, the porousity of the felt discs 96, and the mass of the transducer which may be controlled by the discs 108. These relationships assume that the front shock mount 14A'is highly compliant with respect to deflection along the axis of the microphone casing 12A. 12A. For this reason, a deep circular recess 130 is provided on the side adjacent to the hemispherical end 52 of the casing so that only a thin web 10A against axial deflection.

The volume of the variable volume cavity 42A is responsive only to the component of a force exerted upon the casing 12A which is normal to the diaphragm 28A. Forces exerted perpendicular to this axis (that is, perpendicular to the axis of the casing 50), are not transmitted to the wall in a direction which will change the volume of the cavity 42A, but rather are absorbed by the compliant ring 14A, the stem 122, and the compliant wall 120 in part and in part result in translation of the transducer 10A within the casing 12A. Hence, the variable volume cavity 42A is coupled to the casing 12A by a force resolving coupler which transmits only forces perpendicular to the diaphragm to the flexible wall 120. It is necessary that the volume of the variable volume cavity 42A respond only to the component of forces impressed upon the casing 12A normal to the diaphragm and that the volume of the cavity 42A respond thereto in the same direction as the volume of the diaphragm chamber 46A, that is, both cavities must tend to increase or decrease responsive to the force impressed upon the casing 52A.

The transducer 10A is capable of mechanical resonance within the casing 12A since it has a mass and it is compliantly mounted by the ring 14A and the shock absorber 116. In order to prevent the mechanical resonance of this structure from adversely affecting the shock and vibration isolation, the frequency of resonance is established below the response range of the microphone. The mechanical resonance of the transducer 10A with respect to the casing 12A may be lowered by either increasing the mass of the transducer 10A or increasing the compliants of the mounting structure for the transducer 10A, namely, the ring 14A 'and the shock absorber mount 116.

From the foregoing specification, may modifications of the present invention will be apparent to those skilled in the art, and many advantages not specifically set forth. For example, mechanical vibrations may be impressed upon the transducer, rather than acoustical vibrations without departing from the present invention. Hence, it is intended that the scope of the present invention be not limited by the foregoing specification, but only by the appended claims.

The invention claimed is:

1. A microphone having a frequency response range comprising a hollow casing having an opening at one end thereof, a transducer disposed within the casing having a housing with a cavity therein and an orifice extending from the cavity to the exterior of the housing, said transducer having a diaphargm acoustically sealed about the orifice in the housing with an exterior side confronting the opening in the casing and an interior side, means including the housing and the diaphragm forming a diaphragm chamber on the interior side of the diaphragm, compliant support 'means for mounting the housing on the casing for translation with respect to the casing including a compliant member mounted on the casing and on the confronting surface of the housing, the effective mass of the transducer and'the compliance of the support means forming a mechanical resonance at a frequency below the response range ofthe microphone, the housing includ-' ing means defininga variable volume cavity and a passageway acoustically coupling the cavity with the diaphragm chamber, a force resolving coupling mounted between the casingand the means defining the variable volume cavity for transmitting only the component of accelerations impressed on the casing normal to the diaphragm to the variable volume cavity defining means, the volume of the variable volume cavity changing in the same direction as the diaphragm chamber responsive to forces impressed upon the casing to produce a pressure change in the variable volume cavity which is transmitted through the passageway to oppose the change in volume of the diaphragm chamber.

2. A microphone comprising the combination of claim 1 wherein the means defining the variable volume cavity includes a flexible wall in the form of a second diaphragm disposed generally parallel to the aforesaid first diaphragm, said second diaphragm being mechanically coupled to the casing.

3. A microphone comprising the combination of claim 2 wherein the second diaphragm is disposed on the opposite side of the housing from the first diaphragm and constructed of compliant material, said second diaphragm being coupled to the casing by a rigid-pin affixed to the casing at one end and the diaphragm at the other end. i

4. A microphone comprising the combination of claim 3 wherein the coupling means for transmitting the component of accelerations impressed on the casing normal to the diaphragm to the flexible wall comprises a compliant ring mounted about the housing and on the casing, said ring being disposed in a plane parallel to the first and second diaphragms and between the first and second diaphragms.

5. A microphone comprising the combination of claim 1 wherein the passageway acoustically coupling the cavity with the diaphragm chamber contains means for reducing pressures transmitted from the variable volume cavity to the diaphragm chamber.

6. A microphone comprising the combination of claim 5 wherein the means for reducing pressures comprises an elongated channel in the passageway of a diameter less than the cross section of the cavity.

7. A microphone comprising the combination of claim 6 wherein the means for reducing pressures comprises a second cavity in the passageway with a cross section greater than the elongated channel.

8. A microphone comprising the combination of claim 5 wherein the means for reducing pressures comprises a mass of porous material disposed in'the passageway.

9. A microphone comprising the combination of claim 5 wherein the transducer comprises a voice coil mounted on the diaphragm and a magnetic structure forming a gap accommodating the voice coil and a magnet, the passageway including the gap inthe magnetic structure.

10. A microphone having a frequency response range comprising a hollow tubular casing closed at one end and having an aperture at the opposite end thereof, a transducer having a housing with a cavity therein and an orifice extending from the cavity to the exterior of the housing, said transducer having a diaphragm acoustically sealed about the orifice in the housing with an exterior side confronting the opening in the casing and an interior side, the interior side of said diaphragm being one wall of a diaphragm chamber, an air impermeable shell rigidly secured to the hollow tubular casing, said shell having a recess extending therein from an orifice, the housing of the transducer being disposed within the recess of the shell and extending from the orifice thereof and exposing the diaphragm, said housing only partially filling the recess of the shell and forming a chamber between the housing and the shell, compliant support means for mounting the housing on the shell for translation with respect to the shell only along the axis of elongation of the casing including a compliant member mounted on the shell and on the confronting surface of the housing,

at a frequency below the response range of the microphone, said housing having an aperture extending therethrough on the side of the housing opposite the orifice and said aperture communicating at one end thereof with a recess in the housing of the transducer quency response range of the microphone impressed upon the casing translates the housing with respect to the shell to change the volume of the cavity, thus altering the pressure on the interior side of the diaphragm to oppose shock or vibration induced changes in the position of the diaphragm due to inertia.

' *z'gxga UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,766, 33 Dated October 16, 1973C Inventor) Alan Reed Watson I: is certified that error appears in the above-identified patent and that-said Letters .Iatent are hereby corrected as shown below:

The address of the Assign ee ShOllld be shown as Buchanan, Mich.

Column 1, line 60,

after "disclosed" change "is" to -in Column 3, I line 40, I I

afiter. "material 34 delete "32,". Column 5 line 58 I after "the" (first occnrrence) change "s ideof" to side of-. .v Column 61,; line ,2 8, I i "after "the" -(first .occu rrence) delete "discs". Column 6,; ine 29' v I l I after "is", delete "the". Column 6, line 45 i i after "12 delete "12A. Colnmn 6, line 48 after "web", insert -l32 supports the microphone transducer,.

Column 7, line 16' after "specification", change "may" to -many.

Signed and sealed this 16th day of July 1974.

(SEAL) Attest:

MCCOY IYI. eiBsoN, JR. 51 MAR-SHALLC'DANN At 8 Offlcer I Commissioner of Patents 

1. A microphone having a frequency response range comprising a hollow casing having an opening at one end thereof, a transducer disposed within the casing having a housing with a cavity therein and an orifice extending from the cavity to the exterior of the housing, said transducer having a diaphargm acoustically sealed about the orifice in the housing with an exterior side confronting the opening in the casing and an interior side, means including the housing and the diaphragm forming a diaphragm chamber on the interior side of the diaphragm, compliant support means for mounting the housing on the casing for translation with respect to the casing including a compliant member mounted on the casing and on the confronting surface of the housing, the effective mass of the transducer and the compliance of the support means forming a mechanical resonance at a frequency below the response range of the microphone, the housing including means defining a variable volume cavity and a passageway acoustically coupling the cavity with the diaphragm chamber, a force resolving coupling mounted between the casing and the means defining the variable volume cavity for transmitting only the component of accelerations impressed on the casing normal to the diaphragm and to the variable volume cavity defining means, the volume of the variable volume cavity changing in the same directIon as the diaphragm chamber responsive to forces impressed upon the casing to produce a pressure change in the variable volume cavity which is transmitted through the passageway to oppose the change in volume of the diaphragm chamber.
 2. A microphone comprising the combination of claim 1 wherein the means defining the variable volume cavity includes a flexible wall in the form of a second diaphragm disposed generally parallel to the aforesaid first diaphragm, said second diaphragm being mechanically coupled to the casing.
 3. A microphone comprising the combination of claim 2 wherein the second diaphragm is disposed on the opposite side of the housing from the first diaphragm and constructed of compliant material, said second diaphragm being coupled to the casing by a rigid pin affixed to the casing at one end and the diaphragm at the other end.
 4. A microphone comprising the combination of claim 3 wherein the coupling means for transmitting the component of accelerations impressed on the casing normal to the diaphragm to the flexible wall comprises a compliant ring mounted about the housing and on the casing, said ring being disposed in a plane parallel to the first and second diaphragms and between the first and second diaphragms.
 5. A microphone comprising the combination of claim 1 wherein the passageway acoustically coupling the cavity with the diaphragm chamber contains means for reducing pressures transmitted from the variable volume cavity to the diaphragm chamber.
 6. A microphone comprising the combination of claim 5 wherein the means for reducing pressures comprises an elongated channel in the passageway of a diameter less than the cross section of the cavity.
 7. A microphone comprising the combination of claim 6 wherein the means for reducing pressures comprises a second cavity in the passageway with a cross section greater than the elongated channel.
 8. A microphone comprising the combination of claim 5 wherein the means for reducing pressures comprises a mass of porous material disposed in the passageway.
 9. A microphone comprising the combination of claim 5 wherein the transducer comprises a voice coil mounted on the diaphragm and a magnetic structure forming a gap accommodating the voice coil and a magnet, the passageway including the gap in the magnetic structure.
 10. A microphone having a frequency response range comprising a hollow tubular casing closed at one end and having an aperture at the opposite end thereof, a transducer having a housing with a cavity therein and an orifice extending from the cavity to the exterior of the housing, said transducer having a diaphragm acoustically sealed about the orifice in the housing with an exterior side confronting the opening in the casing and an interior side, the interior side of said diaphragm being one wall of a diaphragm chamber, an air impermeable shell rigidly secured to the hollow tubular casing, said shell having a recess extending therein from an orifice, the housing of the transducer being disposed within the recess of the shell and extending from the orifice thereof and exposing the diaphragm, said housing only partially filling the recess of the shell and forming a chamber between the housing and the shell, compliant support means for mounting the housing on the shell for translation with respect to the shell only along the axis of elongation of the casing including a compliant member mounted on the shell and on the confronting surface of the housing, said compliant member extending about the orifice and of the shell and sealing the orifice of the shell on the housing to acoustically seal the chamber within the shell from the exterior of the shell, the effective mass of the transducer and the compliance of the support means forming a mechanical resonance at a frequency below the response range of the microphone, said housing having an aperture extending therethrough on the side of the housing opposite the orifice and said aperture communicating at one end thereof with a recess in the housing of the transducer opening into the chamber within the shell and at the other end thereof with the diaphragm chamber of the transducer, a compliant membrane extending over the recess and acoustically sealed on the housing about the recess to form a cavity, and a rigid anchor mounted on the shell and affixed centrally of the membrane, whereby a mechanical shock or vibration in the frequency response range of the microphone impressed upon the casing translates the housing with respect to the shell to change the volume of the cavity, thus altering the pressure on the interior side of the diaphragm to oppose shock or vibration induced changes in the position of the diaphragm due to inertia. 